NEW YORK—Even while the cause remains unknown, a deadly blast that leveled two buildings served by a 127-year-old gas main has provided a jarring reminder of just how old and vulnerable much of the infrastructure is in New York and many other cities nationwide.

A detailed report issued only a day before Wednesday’s explosion in East Harlem estimates that $47 billion is needed for repairs and replacement over the next five years to spare New York from havoc.

Nationally, the projected bill—for bridges, highways, mass transit and more—is almost incalculable. Just upgrading the nation’s water and wastewater systems is projected to cost between $3 trillion and $5 trillion over the next 20 years, according to the Competitive Enterprise Institute, a conservative Washington think tank.

Politicians often shy away from blunt talk about infrastructure, but it was in the spotlight Thursday as investigators sought to determine how and why a suspected natural gas leak triggered the explosion, which destroyed two apartment buildings, killed at least eight people and injured more than 60.

The gas pipe serving the building included a cast iron section dating from 1887, and a nearby water main was built in 1897. Federal investigators said the water main broke but it was unknown if that contributed to the gas explosion or was caused by it, and it was unknown whether the gas pipe played any role in the explosion. It was nonetheless upsetting for some New Yorkers to be reminded that Consolidated Edison, the natural gas supplier for East Harlem and much of the rest of the city, makes extensive use of 19th-century piping.

“I can’t imagine how we can have pipes underground in New York that were put in there in the 1800s,” said U.S. Rep. Charles Rangel, a Democrat who represents Harlem in Congress. “You know we talk about infrastructure but the whole damn city is falling apart.”

Mayor Bill de Blasio, who took office on. Jan. 1, says the burden lies with the federal government to provide more aid to U.S. cities for repair and replacement of aging infrastructure.

“The broader infrastructure challenge is something we address every single day with the resources we have, but that is a tough battle considering we are not getting some of the support that we deserve,” he told reporters.

Just Tuesday, a New York-based public policy think tank, the Center for an Urban Future, released a detailed report about New York’s infrastructure, saying it posed problems that “could wreak havoc on the city’s economy and quality of life” if left unchecked. It estimated that $47.3 billion would be needed over the next five years to make crucially needed repairs and replacements.

The report’s author, Adam Forman, noted that Michael Bloomberg, New York’s mayor from 2002 through 2013, oversaw significant new construction, but said the city lost ground during that period in terms of infrastructure maintenance.

“Repairing and replacing aging infrastructure is not glamorous, but it’s critical,” said Forman, who suggested that the East Harlem explosion might be the sort of catalyst needed to gain politicians’ attention.

Forman said the federal government will need to help with the repair bill, but his report also suggests that New York could find some of the needed money through a residential parking permit program and the raising of tolls on East River bridges linking Manhattan with Queens and Brooklyn.

According to Forman’s report:

—More than 1,000 miles of New York City water mains are 100-plus years old. The typical water main is 69 years old, and there have been more than 400 water main breaks annually in recent years.

—More than 160 bridges across the city’s five boroughs were built more than a century ago, and 47 bridges in 2012 were deemed structurally deficient and prone to collapse.

—The subway system abounds with signals that have exceeded their 50-year useful life, slowing the movement of trains and forcing maintenance workers to build their own replacement parts because manufacturers no longer make them.

Similar problems beset cities across the United States.

In Washington, D.C., for example, a research team from Duke University and Boston University recently reported finding more than 5,890 leaks from aging natural gas pipelines. The team said some manholes had methane concentrations about 10 times greater than the threshold at which explosions can occur.

Nationally, the Department of Transportation, which oversees pipelines, estimates that more than 30,000 miles of decades-old cast iron pipe are still being used to deliver gas. A federally monitored replacement program, at a cost in the billions of dollars, is underway but moving slowly, even as occasional tragedies underscore the urgency of the problem.

In 2011, for example, there were two fatal explosions in Pennsylvania linked to old cast-iron mains—one installed in 1928, the other in 1942.

Con Edison, like its counterparts across the country, has a program to replace cast iron pipelines with plastic pipes, costing about $110 million a year. It is now accelerating the program from 50 miles of pipe a year to 65 miles annually, but even at that rate completion could be two nearly decades away.

According to federal data, Con Edison had 1,418 miles of old pipeline to replace in 2004, and as of 2012 still had 1,286 miles left to go.

A Con Ed spokesman, Bob McGee, said the age of the cast iron pipes isn’t a safety hazard in and of itself. But safety experts say the iron pipes are susceptible to cracks caused by earth movement related to seasonal frost heaves, nearby construction work, or changes in ground water levels.

“If I lived near an 1887 small-diameter cast iron main, I’m living next to a ticking time bomb,” said Bob Ackley of Southborough, Mass, a natural gas safety consultant who operates Gas Safety USA. He said utilities should speed up their replacement timetables.

According to the Department of Transportation, New York City still uses about 3,000 miles of decades-old cast-iron gas pipe, Boston about 2,000 miles and Philadelphia about 1,500 miles.

In addition to cost, one deterrent to massive pipe replacement is disruption to traffic on the streets overhead.

Adam Forman of the Center for an Urban Future acknowledged this was a challenge in a crowded city such as New York, but said new technologies were being developed to reduce the need for cutting streets open.

Overall, said Forman’s report, it’s in New York’s self-interest to invest in upgrades of all types.

“Too much of the city’s essential infrastructure remains stuck in the 20th Century—a problem for a city positioning itself to compete with other global cities in today’s 21st Century economy,” he wrote.

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